Non-destructive inspection (NDI) of structures involves thoroughly examining a structure without harming the structure or requiring its significant disassembly. Non-destructive inspection is typically preferred to avoid the schedule, labor, and costs associated with removal of a part for inspection, as well as avoidance of the potential for damaging the structure. Non-destructive inspection is advantageous for many applications in which a thorough inspection of the exterior and/or interior of a structure is required. For example, non-destructive inspection is commonly used in the aircraft industry to inspect aircraft structures for any type of internal or external damage to or defects (flaws) in the structure. Inspection may be performed during manufacturing or after the completed structure has been put into service, including field testing, to validate the integrity and fitness of the structure. In the field, access to interior surfaces of the structure is often restricted, requiring disassembly of the structure, introducing additional time and labor.
Among the structures that are routinely non-destructively tested are composite structures, such as composite sandwich structures and other adhesive bonded panels and assemblies and structures with contoured surfaces. These composite structures, and a shift toward lightweight composite and bonded materials such as using graphite materials, create a need for devices and processes to ensure structural integrity, production quality, and life-cycle support for safe and reliable use. As such, it is frequently desirable to inspect structures to identify any defects, such as cracks, discontinuities, voids, or porosity, which could adversely affect the performance of the structure. For example, a defect in composite sandwich structure comprising one or more layers of lightweight honeycomb or foam core material with composite or metal skins bonded to each side of the core, may include disbonds which may occur at the interfaces between the core and the skin or between the core and a buried septum.
Advanced composite materials, such as carbon fiber composites, are increasingly used in the manufacture of aircraft. Carbon fiber composite panels may be used to create, for example, the fuselage, the nose, and/or the wing structures of aircraft. Carbon fiber composite panels are typically created by combining layers of carbon fiber with a resin, such as a polyester epoxy resin, that binds the fibers together. The resin is typically heat cured, at temperatures and for lengths of time that are precisely predefined. All materials have acceptable operating ranges or envelopes for their properties. Composite materials in general and more particularly composite materials used for aircraft surfaces and structures have defined operating envelopes for their properties. Materials satisfying the envelope criteria are acceptable for use. Materials that exceed the envelope criteria may need to be refurbished, repaired or replaced. Material properties can be altered by any of a number of factors. Heat based influences include fire, lightning, or exhaust impingement. Mechanical influences can include actual impact as well as other stresses to the composite structure such as pressure. Environmental degradation due to sunlight, rain, or salt spray may also occur.
When composite sections are repaired in place (i.e., without removing the damaged or degraded section from the aircraft) and when a particular part warrants additional inspection, it may be desirable to inspect the repaired material to ensure that the properties of the resulting material are within the operating envelope. Inspection may help insure that the repaired composite material as well as the original manufacture of the composite material and their constituent resins were cured in accordance with specifications. In addition to carbon fiber composites, other composite materials, such as fiberglass and ceramic composites, may also benefit from inspection for material property changes due to for instance heat degradation or mechanical stress and/or improper curing during repair.
Current methods exist to detect discrete damage in composite materials. However, existing methods of non-destructively detecting heat degradation are only capable of detecting heat degradation to the surface of the composite structure. In order to measure heat degradation that may exist below the surface, the plies of the composite structure need to be removed sequentially in order to determine how deep the damage is.